Carburization of iron and steel



Jan. 14, 1958 7n??? Wraak/77mm 5 Y im, u j ,A g a WM -5W W o m M ----Ilf, -I Iii H Y 2, B ---JW m llll l- |12 -5 6. IU a@ 1 i.- I@ 11111 6M FM u I Ille NM -i o M l m 0 j Y e *i 2,819,992 United Srtes Patent O r16 aan... .1....14-,1958

fthat carburization `:could: be .carried .out `with fused fsa-lt baths primarily composed of alkaline .earth metallhalides 2819992 'and cyanides ,at temperatures .considerably .higher than CARBURIZATION F RON AND STEEL previously Acustomarily .used with increased'v velocity. and 5 y.intensity of.ithe carburization-if care.` is Staken ifthat the Carl Albrecht Kmberg Germany assgmrlto geutshe alkaline earth metal halideacontentin such zbaths :is .zcon- "'gggllfugmlgg'inasmu` 'mmm s cess er .'-stantly :maintainedgabove '75% andfpreferably atitabout 85%: .Thefcyanidetcontent .ofsuch bathsacarr-becabout 1 t' :Application April 27, 11956, Serial PIU-5815126 the same as.in:^:ithe1fnormal .taceel'eratedtlbatha ithat is,

'.saroundx1057c,.. ltawas unexpectedly Yfound' that .w'ith the :iba-thsfaccording tozitheirimventionafeven whenltemp'eratures ..3C1aims, .((;1 .148 15.5) '.-ofsabove `950 C fornexample 11000 to '1ll0'0'.sC.,:\ar-e employed v for;theicarburizatioml:the.l deleteriousnfdecomfposition of thescyanide is '.nowgrea-ter than with the normal .bathszcontaining 50'f'to 55% foflabarium chloridetoperate'd .atetemp-eraturesriup!to'f9-30 fllhefsrnallzlosses `of iseyanide.occurringduring ause :.o'fithev baths-according to e@he-:invention canfbecornpensated for.: by replenishing 'the 1 `baths with ifa'fzsalltf mixture' fof -corresponding composition. fllhenadvantagesuof th'eltbaths saccordinglto lthew'invention yflieet'lnreilespeciallmapparent: wheni carburizationftemperaturesffbetw'eenfabout 1000 mand lil00 f- =C.f sare*y employed. zu afxpediently; 't-salt mixtures '.:consi-sting'of! 75 to `,95% fof -alkaline .earth metal: halide, :especially :barium `chloride, .Pand 25 tol.5a%tof sodium cyanideareemployedatofprepare :.ithe; bathsorizcarrying" out :the carburization Jprocess acrcordirrgzitoather tinrventionx. orsexamfple,fwhef1 a saltJ rni-xture composed of 90% barium chloride iandk l`0% -lfof 1sodiumefcyanide;1was1 ifuseduand fusedtior carburization of :ironfor steel, itfassutmedith'eiifollowinglfcompositibnafter continued use with replenishment with a mixture off4 par-ts- Vofbarium chloride -and l -partof-sodium--cyanidez Claimsaprioritwapplication Germany April` 27, 1955 l i :'lhe present invention. relatesto` improvementsin Vthe 15 carburization offerrous metals, such..as fironfand steel,

n-aisal-t bath Yessentially consisting of hal-idesl and cyanides, especially at-temperatures higher than previously Qustonlarilycmployedfandto salt baths forcarrying out suehprocess. L.'-l`l're.cemcntation of iron/or steel :articles in fused salt baths containing cyanidesto carburizc .such articles to a `Y.certain depth fbeneat-h their surface is.-already-known. The. fused salt baths .used for this purpose-.contain alkali -metal;and/or alkaline -earth metal -halides and-,one or more `cyanides Yas their essential constituents. The decrease4 of thefcarboncontent with increase-inldepthlfromthe surface of the article carburized and the required duration of the carburization treatment primarily .depend .upon ,the cyanide .content `of the bath employed, the presence of -accelerators and also the .tempera-ture em' .ployed-iformhe.carburization. With baths Vwhichzdo` not f'contai-n laccelerators which generally contain. more-than lower and usually is about 1.0% 'and barium :chloride in 40 In contrast, a known bath for carbufiz a quantity of about to 55% is the main :constituent of A #her'e'naftert'erme'd 'C-5 =bath, :hadthie the :salt bath. The customary vworkingtemperature of catedcompositin fight ftomthe start such accelerated4 baths is 930 C. and it `iszimpossible in 4 v y general to employ them at higher temperatures to accelerate thecarburization .andthe depththereof, .as cyanide 453335,12 i 4 ,"L 5.4 losses whicharenotfeasible occur/atv higher temperatures.. ""clz' v n' 'l 3:5 The cyanide losses occur by burning off and, more irnaCN' I .i j "u :954 jportantfby decomposition 'ofthe cyanide with'the for- "lIaCQa l .Y 1'4 mation fof alkalinecompounds, especially sodium car- C1 j i "Ms Ibonate and-sodium' oxide. These-decomposition;products 50 'KCI- t" "20 evidently deterthe-decomposition of the cyanide `which v @on fis required for the cernenting Vaction and can upon ac- IWGOTIHUedUSWfSCh a'bathf mUSVbIePeShd cumulation in the bath `hinder a satisfactory course of -VWilh'fm"adIIlXtUT'f2133118'Off"IHXffle Same the cementation even though the bathscontainfsuicient '-COIDPOSO aS l'h011"gillbath and-"l'..;p'art. f,.s`0`dium cyamde, -eyanide.'- Consequently, considerably; more 'sodium cya- Many attempts have been made to overcome this diiii-A id'e"-S felfdfof YeP'IEHS'IghS kIIOWh ,lath' .than

cnlty-.by adding accelerators .of the mostyaried'types to for the bath according to the.invention where, for ex- ,the.baths,. for example, compounds of-rnetals, such as aml'l'asesbedbramixture0f4.PI'Sdfbillm their halides, lwhich form .cyandes .which .decompose lXChlOlldfalld"1'paT'OfSdltlmlcyalfde Callemplyd. more readily than alkali metal cyanides. All of these 50"After`C1tin11d"'0peration witlitleC-"ba'fl at `9'3`0 C., attempts, however, have not led to carburization proc` efolsewblelllaltesfof deleteriousnsodiumcarbonate esses which can be carried out at temperatures above "'Cfmcumu 3115 Can amOunt t0 24.2% with a barium 930 C., even though the carburization velocity and depth Chl-Ofdwflellt 0f 50% and a Sodium Cyanide COIleIlt was not always satisfactory. The high cyanide losses, "0f`1'9% as well as the accumulation of carburization detering G5 v"YCSCIEFQIH (the. advantages 0f simpler COmPOSOllfOf decomposition products which cannot be completely com-1- @the baths CCOfldLlgO theiID/.entin .andmore economical pensated for by continued replenishment of the baths NSCNf Teljlenshnglreqlli'emems, it W211S"`1`I11ltlv the with salts, have prevented technicians from attempting "baths according 'to' the 'inventioculd'be operated 'at to carburize with fused cyanide containing salt baths at 7 temperatures over 950 C., for example, between 1000 temperatures in excess of 930 C. worth mentioning. and ll00 C., without undue cyanide losses and elect According to the invention, it was unexpectedly found an unexpected increase in carburization velocity and CaSe lto the invention and with the C5 bath.

Fig. 2 of the drawings shows agraph giving the carbon content at various depths of the case obtained with a bath according to the invention and with the C-5 bath.

Referring to Fig. 1, curves A, B and C represent the carburization depth, corresponding to` a 0.3% carbon contentobtained with test rods of 0.5% C-steei. Curve A represents the results obtained with the C- bath operatedat 930 C;, whereas curves B and C represent the results with a bath according to the invention of 90% BaCl2 and 10% `NaCN, hereinafter termed bath C-8, operating respectively at 1050 C. and 11.00 C. As can be seen from curve A, a carburization depth greater than 2.5 mm. can heardly be expected using the C-5 bath Carhurlzation time in hours Bath Temp.,

vC- 93o 1. s. 1. 25 C-S 1, 050 i. 1. 9 3. 05 3. 90 v0713...... 1,100 1. 65 2.5 4.4 s. 7o

As shown insuch table, the process according to the inventiony renders it possible to obtain carburization depths which were not considered possible with the previously employed processes with the customary baths. Therelationship of the carboncontent with increase `lin carburization depth also indicates the superiority of the process and salt baths according to the invention.

In.Fig...2,.curves D, E and VF represent the carbon contentatvarious depths in the case obtained after 4 hours Vcarburization of test rods of C-l5 steel. tentsat such .variousdepths were ascertained by machinvingoff .successive layers of the case and subjecting the resulting-vturnings to chemical analysis. r'esents the results obtained with the C-S bath operated l at 930 C. vCurves E and F represent the results ob- The carbon con- Curve D reptained'with bath C-8, respectively operated at 1050 C. e

and 1100 C. K

, -The following table gives the carbon content at depths of 1 mm. and 2 mm. taken from the curves of Fig. 2:

Carlini-ization depth Bath Temp.,

1 mm. 2 mm.

Percent 0. 4l 0. 68 0. 78

Percent These values clearly indicate the unexpectedly high carburization action which is obtained with the barium chloride rich baths according to the invention. For example, in the relatively short carburization of 4 hours duration, the same carbon content as is achieved with the known C-S bath at 1 mm. depth is achieved at twice the depth with bath C-B.

, The process according to the invention, especially when employing higher carburization temperatures, has been found particularly suited for the carburization of ferrous metal articles subject to wear wherein a great depth of hardness is desired, for example, for mining chains, caterpillar tractor and excavator parts, and also parts of large engines, such as locomotives and marine diesels.

It is, of course, to be understood that the salt mixtures employed as carburizing baths according to the invention can lcontain other usual additions, such as accelerators or carburization regulators, in addition to the barium chloride and sodium cyanide. However, accelerators are usually not required in view of the higher carburization temperatures which can be employed.

The process according to the invention is advantageously carried out in furnaces provided with ceramic lined containers for the fused salt baths. It has furthermore been found advantageous to provide arched pressure resistant surfaces on the sheet metal cladding for the ceramic lining, especially on the covering plate for the ceramic lining, in order to prevent the ceramic bricks from 30 being lifted up by the salt melt penetrating the joints between the bricks, especially during the heating up or cooling ot of the salt baths.

I claim:

l. A method of carburizing ferrous metals which comprises introducing such ferrous metals into a fused salt bath essentially consisting of a fusion of a mixture of to 95% of barium chloride and 25 to 5% of an alkali metal cyanide maintained at a temperature of at least 1000o C.

2. A method of carburizing ferrous metals which comprises introducing such ferrous metals into a fused salt bath essentially consisting of a fusion of a mixture of 75 to 95% of barium chloride and 25 to 5% of a sodium cyanide maintained at a temperature between 1000 C.

l and 1100" C.

3. The process of claim 2 in which said fused salt bath contains at least -of barium chloride.

References Cited in the file of this patent UNITED STATES PATENTS 1,980,152 Beck et al Nov. 6, 1934 2,007,136 Albrecht et al. July 2, 1935 2,049,806 Holt Aug. 4, 1936 2,339,223 Holt Ian. 11, 1944 2,342,104 Holt Feb. 22, 194

FOREIGN PATENTS 204,759 Great Britain Oct. 1, 1923 OTHER REFERENCES Molten Salt Baths for Heat Treatment and Case Hardening of Steel, published by E. I. du Pont de Nemours & Co. (Ine), lune 7, 1946, pp. 33, 34. 

1. A METHOD OF CARBURIZING FERROUS METALS WHICH COMPRISES INTRODUCING SUCH FERROUS METALS INTO A FUSED SALT BATH ESSENTIALLY CONSISTING OF A FUSION OF A MIXTURE OF 75 TO 95% OF BARIUM CHLORIDE AND 25 TO 5% OF AN ALKALI METAL CYANIDE MAINTAINED AT A TEMPERATURE OF AT LEAST 1000*C. 